Validation of insulation, ratio and burden at rated voltage requires large and heavy test equipment used in conjunction with expensive instrumentation. For this reason, these tests are commonly used during the manufacture of CCVTs but are impractical in the field. Nevertheless, field measurements of the CCVT burden, as an aid to ensuring that the burden rating of the device is not exceeded, are possible. Go/no-go tests are an invaluable part of every maintenance regimen. Options include insulation resistance, leakage current, polarisation index and step voltage testing, and there are few who would argue against the benefits of these core electrical tests. However, these tests will not reveal one of the most common early initiators omotor faults: inter-turn insulation breakdown. To detect such a breakdown, it is necessary to use a surge test. Unfortunately, this is the area where some confusion has been generated in relation to the ‘high’ voltages involved. FIGURE 4: Surge test voltage vs the dielectric strength of a 460 V motor and typical voltage spikes it sees in service The EMU, in addition to an inductive voltage transformer, contains a tuning circuit and protection against ferroresonance (Figure 1). The tuning circuit is a reactor that compensates for magnitude errors and phase shift caused by the CVD, making it possible to have the CCVT with a characteristic on the secondary side that is similar, in terms of error and phase deviation, to that of a purely inductive voltage transformer. Works below the tipping thresholds of RCDs, RCBOs and safety breakers when testing circuits between phase and earth

When the probes of a voltage tester are connected to two points in an electrical circuit (commonly one end to a suspected live conductor and the other to a ground source), a closed circuit or loop is formed. This allows electrical current to flow from the higher potential to the lower potential. The Megger TPT420 Two-Pole Voltage Tester is designed to provide electricians & electrical engineers with a simple, intuitive voltage indicator compatible with both AC and DC voltages. This device can measure AC voltage between 12 to 1000 V and DC voltage between 12 to 1500 V. The TPT420 will even continue to alert the user of a present voltage once the batteries have been completely exhausted. Including a continuity function within 0 to 500 kΩ and a frequency range of 40 to 400Hz, this unit can be applied to numerous situations such as automotive, fire alarm & solar/PV systems. When testing between phase and earth on any circuit protected by an RCD, RCBO, and Safety Breaker, the TPT420 is designed to work below the tripping threshold of these devices to help avoid unintentional disconnection. The phase rotation indication test has been simplified, which now avoids the crossing of the test probes. The TPT420 can also perform a single-pole voltage indication test. And it’s the same with a surge test generated by a Baker DX tester. It applies a voltage and rise-time to enable you to see the inter-coil response, but with a signal controlled in voltage, time and energy, so that the impact on the motor is similar to the spikes that the motor receives as a result of typical power-system variations during everyday operation. There are other effects due to filtering of the higher frequency components of the pulse-wave as a result of attenuation by the steel and the coils own inductance. The speed of propagation of the pulse wave is, however, the key aspect that allows the surge test to detect bad turns.At its core, a voltage tester is a simple device designed to indicate the presence of electrical voltage in a system or a component. The underlying principle lies in Ohm's Law. This states that the current passing through a conductor between two points is directly proportional to the voltage across the two points. Megger’s comprehensive set of transformer testing tools provides a complete view of your transformer’s health. The Megger TPT420 voltage tester is designed to provideelectricians and electrical engineers with an easy-to-use voltage indicating instrument. The meter will continue to warn the operator of dangerous test voltages even when the batteries are exhausted. With both LCD and LED displays, the TPT420provides both AC and DC voltage measurement from 12 to 1000 V AC and up to 1500 V DC. In addition, a continuity function within the range 0 to 500 kΩ, is included and for added safety, continuity, and voltage measurements are also accompanied by an acoustic sounder. The Megger TPT420 LCD/LED Two-Pole Voltage Tester is an AC/DC voltage indicator. It can detect and measure voltages between 12 and 1000V AC and 12 and 1500V DC. The detected voltage range is indicated on the LED scale while the numerical reading is displayed on the LCD.

A bad turn will short and this will be shown by a jump in the ringing frequency of the coil (it becomes in effect a ‘different’ coil at this point in the test). Large frequency jumps can be clearly seen on the tester display, but the instrument’s software also uses mathematical analysis to reveal anomalies that are less easy to spot by eye. The instrument has abright, integrated LED torch to helpyou when working in poorly lit environments and, it is safetyrated at CAT IV 1000 V with an IP64 housing, and conforms to the latest IEC/EN 61243-3 and DIN VDE 0682-401 standards. For compliance, GS38 probe tip shrouds are provided as standard, along with a protective storage and carry pouch. For compliance with the highest safety standards the Megger TPT420 arrives with GS38 probe tip shrouds as standard, along with a protective storage and carry pouch. Key Features But what overall voltage is necessary to show up these turn-to-turn faults, and will this voltage be harmful if applied to a fault-free motor? How do the test voltages relate to the dielectric strength of the windings? CCVTs are devices capable of dual function. One function they can perform is to provide highly accurate voltage conversion for measuring devices, protection relays, and automatic control systems, while the other is to couple high-frequency power-line carrier (PLC) signals onto the transmission system for communication and control purposes.

What’s Included?

CCVT construction varies between manufacturers, models and year of fabrication. Knowledge of the construction is critical when deciding what and how to test. As mentioned previously, some CCVTs are equipped with a potential grounding switch located below C1, at the intermediate voltage terminal (IVT), and a carrier grounding switch below C2 at the low voltage terminal (LVT), as shown in Figure 3. In some CCVTs, however - mainly modern types - the low voltage terminal may not accessible. Understanding the construction characteristics and location of the IVT and LVT is therefore important when determining appropriate connections for testing and deciding whether the ground switches need to be open or closed for the required measurements to be made. Transformers are an integral part of the power grid. Their reliability directly impacts the reliability of the grid. The failure of this critical asset can handicap the grid and increase its volatility. Because replacing a high voltage transformer requires planning for many reasons, including long manufacturing lead times that can exceed a full year, it is widely accepted that asset management, particularly of transformers, is a beneficial contribution to the operation of the grid. Continuity function: detects continuity between 0 and 500kΩ and alerts the user by issuing an audible and visual alert Finally, this ergonomic tester is equipped with a bright LED torch, GS38 probe tip shrouds, and an IP64-rated housing. The Megger TPT420 is also supplied with a carry pouch. In some circumstances, the CVD capacitive reactance can resonate with the magnetizing reactance of the inductive voltage transformer and the compensating reactor cores. This unwanted effect is called ferro-resonance and can give rise to large and damaging voltages across the inductive and capacitive elements. To avoid this, a ferroresonance damping circuit is installed in parallel with one of the secondary windings.

In addition to the unit's adaptable auto-ranging voltage and continuity tests, the Megger MET1000 has a built-in current clamp for AC current measurement to 200 A. With true RMS AC measurements, a phase rotation test, and single pole voltage test built in, the MET1000 is a truly multi-purposeelectrical test instrument, ideal for use in commercial, industrial, and domestic environments. It even comes with GS38 shrouds, provided as standard, to ensure compliance with the latest standards. In a surge test, a short-duration test current with a fast rise time – typically around 100 ns – is generated and applied to the motor coil. The test equipment captures the coil’s response, which takes the form of a decaying oscillation – or ‘ringing transient’. If the motor coil were in air, the pulse would be travelling at almost the speed of light and the voltage would be evenly distributed across the coils. But in a motor, the coil is not in air, it is wrapped around a steel core, and so the pulse travels much more slowly. On-Line testing – that is, testing while the motor is running – can also be carried out on an as-needed basis with a portable on-line monitor like the Baker EXP4000.In addition to this, the Megger TPT420 LCD/LED Two-Pole Voltage Tester features a Continuity function which will alert the user to the presence of continuity between 0 and 500kΩ via a visual and audible alert. The Megger TPT420 LCD/LED Two-Pole Voltage Tester will also issue an audible alert upon detection of voltage and will continue to warn the user of dangerous test voltages even when its batteries have expired. This voltage tester has a safety rating of CAT IV 1000V. Capacitance and line frequency power factor (PF) measurements should be made routinely on CCVTs. Insulation power factor tests are most informative when the amount of insulation included in the test is minimised. For this reason, tests are performed on each individual component of the CVD (e.g., C1-1, C1-2, …, and C2). Typical overall PF values range from 0.2 % to 0.5 %, but power factor values under 0.05 % are normal depending on the insulating materials used for construction. A basic ratio test can be carried out by exciting the primary side of the CCVT with a 10 kV source and measuring the secondary voltage with a digital multimeter. However, this supplies no measurement of phase deviation, which is required to validate the accuracy of the CCVT.

Furthermore, the Megger TPT420 LCD/LED Two-Pole Voltage Tester supports single-pole voltage tests and simplified phase rotation indication tests which avoid crossing the test probes. Moreover, when testing circuits between phase and earth, the Megger TPT420 LCD/LED Two-Pole Voltage Tester will work below the tipping threshold of RCDs, RCBOs and safety breakers to prevent unintentional disconnection. As mentioned in the previous article, there are many options for testing insulation, and Megger supplies an extensive range of testers for such applications, from 50 V to 15 kV insulation testers, through VLF and AC Tan Delta test sets to diagnostic Dielectric Frequency Response instruments and, under the Baker brand, specialist motor testing equipment up to 40 kV. Inside the voltage tester, this current passes through a known resistance. The voltage drop across this internal resistance is then measured and displayed on the tester's meter or screen. This provides the user with a reading of the voltage present. In some cases, for basic testers, this voltage may simply trigger a light or a sound to indicate the presence of voltage rather than providing a precise numerical value. To answer these questions, let’s look at an example. When a 415 V motor is assembled, the insulation applied to the wire used to wind the stator has a dielectric strength of approximately 8000 V. During its lifetime, this insulation will degrade primarily due to heat in the motor, but also as a result of environmental conditions and the coil movements which arise from starting, stopping and load changes.

These on-line techniques, of course, cannot tell you about all types of incipient faults. Indeed, when warnings are given, a full diagnosis may require complementary off-line testing. In addition, off-line tests are often used on their own in a planned maintenance regime that takes motors off-line for testing and other maintenance activities at appropriate intervals. Compared with on-line tests, off-line tests provide different insights into the motor’s condition; the two approaches to testing are, therefore, complementary.